Offspring Human Fertility Behavior in Biodemographic Perspective (2003) / Chapter Skim
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3. Education, Fertility, and Heritability: Explaining a Paradox
3. Education, Fertility, and Heritability: Explaining a Paradox
Pages 46-90
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From page 46... ...
is occurring in research on human fertility. Our particular interest is in the interplay between demography and biology, as these two very different disciplines begin to blend in the long-standing effort to develop models and theories to explain human fertility.
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From page 47... ...
The controversy that can occur when disciplinary boundaries are crossed in the consilience process is illustrated by the responses to the van den Oord and Rowe paper in a later issue of Demography (Frank, 2001; Zuberi,2001~.i Similarly, a paper by Morgan and King (2001) that reviews the biological predispositions, social coercion, and individual incentives for having children in contemporary below-replacement fertility contexts resulted in diverging opinions about the usefulness of biodemographic or behavioral genetics approaches in understanding contemporary fertility behavior (Kohier, 2001; Capron and Vetta, 2001~.
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From page 48... ...
· Genetic influences interact with environmental influences in fascinating and subtle ways. For example, Neiss et al.
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From page 49... ...
Multivariate behavioral genetics modeling has the potential to identify overlapping sources of variance in both genetic and environmental domains. Topics of potential interest to demographers include the following: Are the genetic sources of influence on human fertility of the same nature for early fertility, general fertility, and later fertility?
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From page 50... ...
These shifts in socioeconomic conditions have also led to changes in many fertility-related behaviors, such as marriage/union formation and female labor force participation, which are also closely related to changing demands for children. It should be noted that such socioeconomic considerations of fertility change are rarely inconsistent with biological theories.
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From page 51... ...
Variations in individuals' life courses including variations with respect to important fertility outcomes therefore arise for two reasons: First, individuals' desired life courses differ because of different socioeconomic opportunities or constraints and because individuals have different abilities, preferences, physical characteristics, and so forth, that they take into account in making life cycle decisions and plans. Second, the realized life courses of individuals differ from their initial plans, as well as among individuals, due to shocks in fertility and fertility-related behaviors/processes, such as an unexpectedly long waiting time to conception or the death of a spouse.
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From page 52... ...
On the one hand, biological dispositions affect fertility relatively directly through genetically mediated variations in physiological characteristics affecting fertility outcomes. Genetic influences on fecundity are an obvious example (e.g., see Christensen et al., 2003)
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From page 53... ...
A possibly surprising but robust finding in some of our earlier analyses (Kohier et al., 1999, 2002b) , for instance, is a systematic relationship between fertility transitions and patterns in both heri"abilities and shared environmental variance in data on female Danish twins: Increased opportunities for education and labor market participation and the emergence of relaxed and flexible reproductive norms in recent decades seem to have strengthened the genetic component in fertility outcomes.
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From page 54... ...
Future analyses therefore not only need to estimate sophisticated behavioral genetics models for fertility and related behaviors but also need to allow for interactions between the socioeconomic context, individual characteristics and life histories, and patterns of heritability. In the empirical part of this chapter we apply behavioral genetics designs and models to study one such interaction between genetic dispositions and socioeconomic environments, specifically education, and show how the effect of genetic dispositions on fertility differs between individuals with different education levels.
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From page 55... ...
, and nonshared environmental variability (or eel. These parameters represent, respectively, the fraction of variance in a trait or outcome that is due to genetic factors, shared environmental influences like common family backgrounds, and finally individual-specific environmental factors.
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This concern is a form of the well-studied equal environments assumption of behavioral genetics modeling. This assumption has been studied in several ways.
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shared environmental influences are typically smaller than genetic influences on behavior; and (3) a great deal of behavioral variability is not based in either genetics or families.
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From page 58... ...
On the contrary, the understanding of genetic influences on fertility and related behavior outlined in the previous section suggests transformation in the pattern of how genetic shared environmental and individual-specific environmental factors contribute to variations in fertility outcomes in a population. Nevertheless, our previous research was not able to identify specific socioeconomic changes or characteristics that are the driving forces behind these transformations, and our earlier analyses relied on indirect explanation on the basis of overall socioeconomic and demographic changes across cohorts.
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From page 59... ...
The influence of education continues to be prominent also in posttransitional contexts with low fertility because it is an important determinant of female wages, which determine the opportunity costs of childbearing and shift the relative bargaining of males and females in households with potentially important effects on fertility decisions. In addition, pursuing education, particularly higher education, is associated with specific social environments that can affect fertility preferences and desires if social interactions affect fertility decisions through social learning or social influence (e.g., Montgomery and Casterline, 1996; Kohier et al., 2002c)
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From page 60... ...
First, we estimate multivariate behavioral genetics models to decompose the variance in fertility into genetic and shared environmental components and to assess the extent to which there are overlapping sources of genetic and shared environmental influences that affect both education and fertility. Second, we extend the results from Kohier et al.
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From page 61... ...
Education levels in Denmark are relatively high for all cohorts, with more than 60 percent of individuals pursuing some kind of tertiary education. The most important change in female education has been the decline of women with no education beyond elementary education and an increase in women with training at apprentice school.
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From page 62... ...
since this training primarily pertains to the highest education categories in the table, it can be expected that the 1965-1970 cohort attained the highest education levels in the table. Updated information about this completed education will be available soon from new survey data collected in 2002.
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From page 63... ...
In this survey DZ twins were born more frequently to older mothers for the birth years covered. A close congruence between the fertility patterns of twins and those of the general population is therefore an essential precondition in order to generalize the results of twin-based investigations into biosocial determinants of fertility to the general population.
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From page 64... ...
MULTIVARIATE BEHAVIORAL GENETICS MODELS FOR EDUCATION AND FERTILITY Our first set of analyses considers jointly the genetic and shared environmental variance components in completed education and fertility. We restrict these analyses to cohorts born prior to 1963 that is, the subset of cohorts at least 35 years old in 1998 when our fertility data were censored.
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From page 65... ...
The relative importance of these influences is usually expressed in terms of heritability, h2, which is equal to the proportion of total phenotypic variance attributable to (additive) genetic variance, and the coefficient of shared environmental influences, c2, which is equal to the proportion of the total variance related to differences in shared-environmental conditions, such as parental background, and socialization.
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From page 66... ...
The mode! therefore suggests that about 33 percent of the variance in our education measure and 35 percent of the variance in fertility are related to genetic factors, while 18 percent and 2 percent, respectively, are related to shared environmental influences.7 The mode!
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From page 67... ...
Genetic variance in fertility is therefore almost exclusively due to residual genetic influences that affect fertility but not education. The situation is strikingly different for shared environmental influences, where the results in Figure 3-1 reveal no residual shared environmental influences for fertility; all shared environmental influences on fertility result from factors that also affect education, and shared environmental influences that increase education tend to decrease fertility.
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From page 68... ...
in Figures 3-1 and 3-2 confirms our earlier findings that fertility in low-fertility settings, such as contemporary Denmark, is subject to important genetic influences. A new and somewhat unexpected result of the above analyses is that genetic variance in fertility is not necessarily shared with genetic variance in completed education (measured in years of tertiary education)
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From page 69... ...
The bivariate probit model is a special case with only two categories and ~ = 1. To estimate heritabilities, the correlation of the random terms ei1 and ei2 in twin pairs is specified as Pi = 61 + 62Ri, where Ri is the genetic relatedness of twin pair i.
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From page 70... ...
If these opportunities are an important factor affecting the relevance of genetic influences on fertility, as we argued in our earlier studies, our reasoning suggests that primary and secondary education should interact with the genetic etiology of fertility. In our subsequent bivariate probit analyses of the within-M, and within-D, pair correlations in the propensity to have children, we include the above education measure (years of primary and secondary education)
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From page 71... ...
s~ o s~ o · c~ · c~ · o s~ s~ o c~ · s~ c~ .> ~4 c~ c~ 1 ~4 ~; o o o mo ~ o L)
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From page 73... ...
. Focusing on the DZ and MZ twin correlations in our present analyses, instead of heritabilities and shared environmental influences, avoids the need to specify these interactions in more detail at this stage of the analysis.
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From page 74... ...
4) , we allow for the possibility that education affects not only the level of fertility but also the relevance of genetic and shared environmental influences on fertility.
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From page 75... ...
In terms of a heritability mode! with shared environmental and genetic influences, this suggests that higher education tends to increase the proportion of variance attributed to genetic factors and to decrease the fraction attributed to shared environmental factors.
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From page 76... ...
0.55 0.50 c ,o _! ED 0.45 o rat 0.40 1 1 1 1 ~ 1 · o 1 1 / 1 I I f I 1 1 / 1 1 1/ 1 t / 1 / 1 1 / 1 1 ~ Education effect 1 i/ ~ ~ ~ 1 1 / ~ -1 std.
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From page 77... ...
Figure 3-3c presents the results of Mode! 5, which includes interaction with birth year as well as education levels and shows the extent to which the within-M, correlation across the 1953-1970 cohorts increases due to the interaction with birth cohort (holding education constant at 11.04 years)
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From page 78... ...
If the analyses in Table 35 are replicated using this alternative education measure, the results remain unchanged: Twin pairs with higher education levels are subject to significantly higher MZ (but not DZ) correlations in the propensity to have chilciren, and the increase in educational attainment across cohorts partially explains the cohort trenci toward increased heritabilities for fertility in younger cohorts.
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From page 79... ...
In summary, these findings for males are consistent with our earlier findings in Kohier et al. (1999~: There seem to be relevant genetic influences on fertility outcomes also for males, but the social conditioning of these heritability patterns is weaker.
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From page 82... ...
This implies that it is primarily the genetic factors consistent with variation in fertility outcomes that are affected by education and cohorts. In particular, our analyses suggested that genetic influences tend to become stronger in twin pairs with a higher level of education and that genetic influences tend to become stronger in more recent cohorts.
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From page 83... ...
These options increase the set of potential pathways through which genetic influences affect fertility outcome, and increased opportunities for instance in the labor market are likely to heighten the implications of endowments for labor market outcomes and therefore indirectly also on fertility. The theoretical framework of fertility behavior that is embedded in a broad context of life cycle decisions and processes provides a basis for analyzing and understanding these changing contributions of genetic factors to variations in fertility outcomes, and future analyses that combine detailed socioeconomic information and multivariate behavioral genetics models can investigate these different pathways in greater detail.
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From page 84... ...
Nevertheless, because (additive) genetic influences on fertility tend to cause a positive intergenerational correlation (though not necessarily if genetic factors operate through epistatis)
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From page 85... ...
Bean, and G.P. Mineau 1987 Intergenerational transmission of relative fertility and life course patterns.
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From page 86... ...
Owens, and N.G. Martin 2001 Natural selection and quantitative genetics of life history traits in Western women: A twin study.
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From page 87... ...
, reproductive behaviors, and twinning. In Genetic Influences on Human Fertility and Sexuality.
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From page 88... ...
Rowe, and W.B. Miller 2001a Genetic influences help explain variation in human fertility outcomes: Evidence from recent behavioral and molecular genetic studies.
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From page 89... ...
Rowe, and K May 1994b DF analysis of NLSY IQ/achievement data: Nonshared environmental influences.
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From page 90... ...
9o OFFSPRING Wood, J 1994 Dynamics of Human Reproduction: Biology, Biometry, Demography.
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